Understanding water footprint numbers

“There are lies, damned lies and statistics.” Mark Twain

As a society, our culture is enamored with numbers. When cited such stats seemingly give credibility to a person’s arguments. When repeated, such stats also seemingly become indisputable facts. For example, look at water footprint numbers. Who hasn’t heard how many gallons it takes to grow an almond or produce a pound of beef? These stats have been widely disseminated through the media (like in a recent Sunday editorial in the New York Times by Nicholas Kristof) as well as used in numerous debates to further agendas and food ideologies.

 But how many people have taken the time to understand how such numbers and stats are derived? What do the numbers really mean? Are the numbers even accurate? In a stat, number reliant world, often it seems that any given number is accepted uncritically especially when such stats further one’s beliefs. So looking again at water footprint numbers, does it really take a gallon of water for a single almond or 1800 gallons of water for a pound of beef?


Well, the answer with water footprint numbers is “it depends.” Unlike with sport stats like, for example, a basketball player’s free throw percentage, there are a lot more parameters (and limitations) involved beyond simply shots taken and shots made.

To derive water footprint numbers a lot of data is gathered and plugged into water balance models with equations by groups like the Water Footprint Network. Data gathered includes rainfall, evapotranspiration, yield of product (meat, produce, and consumer goods), life times or length of growing seasons, daily and monthly air temperatures, etc. Other factors considered are soil type, and average nitrate use. Though for these last two factors, as well as others, a lot of assumptions are made.

G.I.S. (geographical information system) mapping software (ArcGIS) is used to input data from regions of the world utilizing different production methods. The generic numbers most often cited are theoretically global averages for different products. Though numbers can vary greatly depending upon location and methodology of production. More exact and specific numbers for any area may also be derived from this mapping software by analyzing only the data for specific areas of the grid. As more and more data is entered into this mapping software, better information may be derived from it. Though currently, there are a lot of limitations on these models simply because the natural world is quite complex. Given these limitations, water footprint numbers are often quite meaningless when used to assess environmental impacts especially when trying to compare different products and their respective water use efficiencies.

So, in short, water footprint number calculations look at the total amount of water consumed and used indirectly or directly for a product over the entire lifetime of growing, raising, or manufacturing that product on a per unit basis. Thus very simply put overall yield of any product is divided by water consumed over the time required to generate a per unit number for example an almond, carrot, pound of beef or a tee-shirt.

Additionally in more recent analysis done to generate water footprint numbers, the source of the water is also taken into account divided specifically into green, blue and grey water. Green is rainfall. Blue is surface water including from rivers and aquifers used for well water and irrigation. While grey water is the amount of water needed to dilute polluted water so that water can be reused. In reality though according to Chris Perry in his paper Water footprints: Path to enlightenment, or false trail? “The notions of green and blue water imply that soil moisture, groundwater, and surface flows are separate, distinct, and independent sources of water, whereas they are interdependent components of the same hydrological system.”

Regardless, to better understand this information above, and in particular the limitations of water footprint numbers both in how they’re derived and why they often are meaningless in assessing environmental impacts, it helps to look at examples.

On page nine of Mekonnen and Hoekstra’s 2011 paper “The green, blue and grey water footprint of crops and derived products.” the water footprint numbers are given for different crops including soy, corn and wheat in gallons per metric ton. The numbers for water are further broken down by green, blue and grey water. For most crops, the majority of water comes from green water (rain). Though in regions of the world that are arid, more water will come from surface or blue water. So the mix of the sources of water varies quite a bit based on local climate. More blue water is required in Southern California than in Oregon.

The math and methodologies are also described in this 2011 paper, so one can see that the time for growth, the amount of water, and the yield all factor into the equation. A higher yield will result in a lower water footprint. A shorter growth cycle will result in a lower footprint. A high yield with a short growth cycle provides an even lower number. Currently though, methods of irrigations don’t factor into the equation, though future water footprint analysis according to correspondences with the Water Footprint Network will include this information. Whether mulch is or isn’t used to reduce evaporation doesn’t factor into the number. Multi-crop or poly-crop systems also aren’t assessed. Nor are integrated ones. Soil type is averaged over portions of the grid. Nitrate use, which factors into grey water needed, is averaged by country. So for soil and nitrate use, the modeling isn’t very precise. Given how variable soil is and its impact upon water retention, this is a major limitation of this analytical modeling.

With almonds and other nut or fruit trees, the water needed to grow the tree to the point where it starts to yield nuts or fruits also has to be accounted for in the water footprint number. So all the water required to grow an almond tree for three years before it starts to yield any nuts also is factored into the water footprint number. Thus the lifecycle for a nut is much longer than it is for a carrot which, in part, accounts for a higher water foot print number. Almonds also require water during the processing to remove the shell. Thus an unshelled almond has a lower water footprint number than a shelled one. Almonds, pistachios and other crops requiring irrigation get their blue water from diverted river water or from pumped ground or aquifer water. In a drought, there is very little green water. Land not suitable for crop production converted to cropland requires a lot of diverted blue water.

Where blue water comes from may have a tremendous environmental impact. Even pumping up water from an aquifer 150 feet deep versus one that is 1200 deep on the same land may have a much different impact since pumping up deeper wells pulls up salt and other minerals including toxic ones like selenium and arsenic. Salinity reduces land fertility adversely impacting yields, and selenium kills birds. Water footprint numbers alone give zero insights into these issues and impacts. Thus a low water footprint carrot or head of lettuce grown on non-arable land converted to cropland via diverted or pumped ground blue water may have a much greater environmental impact than simply leaving the land as grassland for grazing livestock beef cattle.

Beef, as every vegan meme creator knows, has a very high water footprint number. This number isn’t high because calves, cows, bulls, steers and heifers are guzzling down drinking water. No this number is high because beef cattle are higher up the food chain. According to Mekonnen and Hoekstra’s 2010 paper “The green, blue and grey water footprint of farm animals and animal products,” 98% of the water required (global average) for beef is for the amount of water needed to grow feed, forage or grasses that the cattle eat. Drinking water, service water and feed mixing water are respectively 1.1%, 0.8% and 0.03%. Furthermore per the global average 87.2% of this water is green water, rainfall, with the remaining balance being 6.2% blue and 6.6% grey water. So the water required for beef is the total sum of all the water (mostly rain) that is needed to grow all the grasses, forage and feed that a head of cattle eats during its entire lifetime from birth to slaughter.

Mekonnen and Hoekstra’s 2010 paper also looks at the production system namely conventional versus pastured systems. With conventional systems where calves as yearlings are transferred from grass to feedlots, the feed itself has a significantly higher foot print number than grasses. However the animal’s life is shorter getting to yield faster due to the grain feed, genetics, and use of growth hormones. Whereas with pastured systems where the cattle is finished on grasses, legumes, and forbs, the cattle takes longer to get to slaughter weight and thus need to consume more food over more time for slightly lower yields. Though with pastured systems, there is very little blue and grey water used to grow the various grasses and forbs.

Also worth noting, cattle on grasslands also return much of what they drink (the 1.1% portion of the footprint) into the soil via urine and manure. The water consumed by pastured cattle is even less than feedlot finished beef since grasses have more water than grains. Cattle on grass get 70 to 90% of their water from the grasses they eat (see page 3, Beef Cattle Nutrition fundamentals from University of Minnesota). Grains provide much less- around 30%- of cattle’s water needs.

Given this analysis, those in the conventional beef industry and, on the opposite side of the spectrum, abolitionist vegans argue that pastured beef production is less efficient and has a larger environmental impact because the cattle require more resources due to the cattle living longer. Mekonnen and Hoekstra in their summary actually make this same assessment. Though in doing their analysis, they make a number of mistakes including not looking at appropriate land use. Their data for water footprint numbers of natural drought resistance grasses is also non-existent. So as noted in the example given above, converting non-arable grasslands used for grazing high water footprint cattle to croplands for low water footprint vegetables is not an efficient use of water resources.

Much of the earth’s land mass is simply not suitable for crop production. This land may be too rocky, too windy, too steep, too dry, have too short a growing season or some combination of any or all of these factors. This land, including vast grasslands, is where most cattle are raised (according to a beefboard.org “fact sheet” in the US 85% of the land isn’t suitable for crop production though like any other number this needs to be confirmed) . Many cattle ranches don’t use any form of irrigation even in Northern California in the midst of a multi-year drought. A sufficient amount of grasses and forbs still grow on these ranches to graze cattle.

How is this possible with so little rainfall?

To begin with grasslands include many perennials with long root systems and provide continuous cover which reduces the surface temperature reducing evaporation. Cattle when properly managed using movement through paddocks aren’t allowed to overgraze. The cattle also spread manure which increases soil microbial activity which both builds topsoil and improves soil health. The domesticated ruminants are essential to these grassland ecosystems like the wild ruminants (bison, elk, deer, and antelope) they replaced. Healthy soils not only sequester carbon and function as methane sinks, they also retain more water. Since they also have higher and faster rates of retention (see video), whatever rainfall that falls is better utilized. The land becomes a “water bank.” These healthy soils also need no grey water to mitigate run-off. Tilled and non-tilled agriculture conversely reduces water infiltration and reduce soil microbial activity. Exposed earth gets hotter, which kills microbes, plus releases CO2 into the atmosphere. This leads to soil compaction especially when used with nitrates and other synthetic fertilizers. Thus more polluted run-off and need for grey water.

So cattle on grasslands offer a more efficient use of the water in those ecosystems than non-appropriate land uses like agricultural ones. This is so because the water that’s used to grow the grasses and forbs the cattle eat is almost exclusively rain (green) water. This water is best utilized by grazing cattle to convert into protein. There isn’t a more efficient use of this rain water. Conversion of non-arable land to cropland as discussed also isn’t an efficient use of blue water. Additionally water used to grow feed crops for conventional feedlot beef cattle finished on grains (soy and corn) also isn’t the most appropriate land use, since that land is arable and can be used for other less crops. (This use of land also makes no sense for growing crops for ethanol). However cattle used in minimally tilled integrated systems as part of crop rotations also build healthy soils where both crops, cover crops, and beef can be produced on the same land (see Gabe Brown’s video on building soil with integrated systems). Again this leads to more optimal use of retained rainfall due to the creation of healthy soils.

The modeling used to generate these numbers can’t and doesn’t account for all these different variables that are part of complex ecosystems. AsPerry notes,”Water footprint analysis, by contrast, is a dis-integrated approach that reflects an over-simplification of complex issues.” Thus the numbers frequently are completely meaningless when assessing environmental impacts. High or low water footprint numbers also aren’t always relevant or as important as appropriate land use when considering what type of food to produce on any given piece of land. Rain – green water- falls on the land regardless of the use, so determining what’s the best use of the land is what’s paramount.

Water footprint numbers potentially could be a very useful deductive tool. But when used without any real understanding of what the numbers mean or how the numbers were derived, the numbers become just more reductive thinking reinforcing the disconnect between the food on our forks and how that food was grown or raised. With such a disconnection, such numbers rather than raise awareness only instead raise ideological bias and further misunderstandings. So, to whom this may concern, please try to make some effort to understand what numbers mean before simply regurgitating them.

Thanks in advance.

(Originally published June 8th, 2015 on Examiner.com)

9 thoughts on “Understanding water footprint numbers

  1. As a vegan, I have to say that you convinced me quite a bit of the potential environmental benefits of using this non-arable land effectively. Thans for explaining the reductionistic view of virtual water when it comes to gras-fed cattle. Are there industries/products where the current standard of blue/green/grey water usage suggested by Mekonnen and Hoekstra are useful in assesing hydrological impact or is it alwasy a non-valid instrument?


  2. 1 litre of water will cover 1 sq metre to the depth of 1mm.
    So 1mm hectare of rain = 10,000 Litres / hectare
    So 175mm (or 7” -fairly typical low rainfall ranchland) is going to get 1,750,000 litres per hectare per year ( or 700 tons of water/acre, same thing)
    The rain will still fall regardless of whether there are animals there or not.
    One thing is certain, the erosion and dust caused by trying to cultivate a crop on this rainfall would ruin it.
    Have some faith in your farmers. We spend our lives trying to work out the best fit for our land, then some a..hole from thousands of miles away says we got it wrong.
    Hell, I don’t even tell my neighbor what he should do unless he asks me, because there will be something different on his place which don’t apply to mine


    1. Usually it’s some a-hole from thousands of miles away who is staring at a computer screen full of data reports that has never set foot farm who says you’re doing it wrong. LOL


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